Rechargeable batteries which may be repeatedly charged are used in electronic devices such as mobile terminals. Methods of charging a rechargeable battery include a method of charging a rechargeable battery through metal contacts, and a wireless charging method of charging a battery without the intermediary of metal contacts or the like. Of these methods, the wireless charging method may contribute to convenience for a user because the user does not have to bring a battery into contact with metal contacts of a battery charger.
In the wireless charging method, a magnetic field is generated by supplying electric power from a power supply to a power transmitting coil, and the magnetic field causes a power receiving coil to generate an induced electromotive force. The power receiving coil is built in a battery, and the battery is charged by the induced electromotive force.
The wireless charging method is further categorized into an electromagnetic induction mode and a magnetic field resonance mode. These modes are distinguished from each other depending on a coupling coefficient between the power transmitting coil and the power receiving coil as well as on a Q factor. Here, a mode involving a small Q factor and a large coupling coefficient is often referred to as the electromagnetic induction mode. In contrast, a mode involving a large Q factor and a small coupling coefficient is often referred to as the magnetic field resonance mode.
According to the electromagnetic induction mode, the power receiving coil and the power transmitting coil have to be located face to face in order to improve power transmission efficiency. In this regard, the electromagnetic induction mode causes a constraint on a positional relation between the power receiving coil and the power transmitting coil, and is therefore inconvenient.
On the other hand, the magnetic field resonance mode is known to be capable of achieving adequate transmission efficiency without locating the power receiving coil and the power transmitting coil face to face. In this regard, the magnetic field resonance mode is a charging mode which is more convenient than the electromagnetic induction mode.
The wireless charging using the above-described magnetic field resonance mode still has room for improvement in light of further increase in the flexibility of the positional relation between the power receiving coil and the power transmitting coil.